Global warming could make individual animals smaller by causing them to mature faster than they grow. As a result, they reach adulthood at a smaller size when temperatures are warm.

CLIMATE CHANGE LAB: A healthy calanoid copepod. Data from many copepods, similar to this one, has given scientist insight into how warming temperatures could cause individual animals to become smaller. (Photo: Wikimedia Commons)

Melting ice, outbreaks of disease, more intense storms and more forest fires are just some of the effects scientists say will accompany human-caused climate change. Scientists are now exploring another, perhaps more surprising, potential effect: Shrinking animals.

A new study has examined how warmer temperatures can result in smaller individuals within a species.

This relationship between size and temperature change only holds for cold-blooded animals, which rely on external sources, such as sunlight, to warm themselves. Scientists don't understand why this relationship exists. But it's important because size influences an individual's reproductive success, as smaller animals tend to have fewer offspring, and its role in a food chain, among other things.

To warm-blooded creatures like humans, this might not sound like a big deal. But we make up only a tiny percentage of Earth's animals, and we rely upon cold-blooded creatures for food, to pollinate crops and for many other crucial, but perhaps not obvious, reasons. So, climate-influenced changes could have cascading effects.

Scientists have already established the "temperature size rule," which says that individual animals reared at colder temperatures will become larger adults. Likewise, animals reared in warmer temperatures produce smaller adults. However, it's unclear just how this happens, according to Jack Forster, a doctoral student at Queen Mary, University of London, and the lead researcher.

Forster and his colleagues looked at data on copepods — tiny, water-dwelling crustaceans — to study what was happening for a range of species. Using data already collected for 34 marine species of copepod, they looked at how non-extreme temperatures influenced growth rate (how quickly an animal puts on weight) and development (how quickly they passed through life stages). For copepods, there is plenty of development to track, since they go through 13 life stages, from egg to adult.

The researchers' analysis revealed that development rate is more sensitive to temperature than growth rate.

"If you warm up, you put on mass more quickly, but the rate at which you pass through life stages is even faster, and when you reach an adult size, you end up being smaller at warmer temperatures," Forster said.

It's not clear why this is the case, he said.

Their analysis also revealed that while the egg did not respond to warmth, the gap between development rate and growth rate tended to widen begining around the second life stage until adulthood. When the animal reached maturity, its final adult size was smaller as a result, they found.

As zooplankton, or tiny, floating animals, copepods are a key component of the ocean food web, so if warming in the oceans prompts these animals to shrink, it could have a direct effect on the things they eat and what eats them. The fish that eat them, for example, will have to spend more time searching for more of them to eat. As cold-blooded creatures themselves, the fish could also be affected by the warming waters, creating a compound effect, which could result in even smaller fish.

It's also possible that the fish could switch to other prey, a move that could have its own ripple effects. However, both of these scenarios are hypothetical, Forster said.

The researchers' previous work has shown that size decreases by an average of 2.5 percent for every 1.8 degrees Fahrenheit (1 degree Celsius) of warming for a range of cold-blooded creatures, including insects, crustaceans, fish, amphibians and reptiles. Some species of copepods have shown larger size changes with temperature.

Cold-blooded animals may not be the only ones affected by temperature changes: There is evidence that the temperature size rule also holds for single-celled protists and in plants, according to Forster.

The research was published online Sept. 29 in the journal The American Naturalist.